The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Phased arrays, and particularly aeroacoustic phased arrays, have become a standard measurement tool for noise engineering. Such phased arrays are frequently used in development tests of various products such as aircraft, and employed in wind tunnels to enable simultaneous aerodynamic and acoustic data acquisition.
The present disclosure is directed to the problem of designing a planar phased array which is useful across a broad range of frequencies, and where the available number of sensors (i.e. microphones) in the array is restricted such that a regular (i.e., equally spaced element) array cannot be achieved with intra-sensor spacing meeting the half-wavelength criteria typically required to avoid spatial aliasing contamination in source maps or projected beams. A particular problem for such planar arrays is where the primary direction for beamforming is substantially off-axis of the array. This is an especially common problem, for example, for aeroacoustic phased array measurements taken in wind tunnels and fly-over noise measurements.
In one example, when the phased array is used within a wind tunnel it is commonly placed along a sideline or wall of the wind tunnel or flat on the ground so that the array orientation is restricted. In such an application, the primary “look” direction will be determined by the position of the model under test with respect to the array position along the wall of the wind tunnel. Beamforming must then be performed off-axis, which reduces the effective aperture of the array. In particular, circular arrays are less effective in beamforming in the off-axis direction and suffer a loss of resolution in the dimension corresponding to the look direction relative to the resolution in the direction perpendicular to the look direction.
In one example, the array may be placed in a stationary position, and the device under test may move relative to the array. In these instances, the off-axis or look angle of the array is defined by where it is desired for the array to look relative to the moving source instead of where the array is moved relative to a stationary source.
It may therefore be desirable to provide a planar array that is particularly well adapted to be used in aeroacoustic applications where off-axis beamforming is required. More specifically, it is a principal object of the present invention to provide a planar array which is especially well suited to performing off-axis beamforming without suffering reduced resolution in the look direction typically experienced with circular arrays in such applications.